In an Ethernet network, all nodes generate frames in the well-known Ethernet format. However, before transmission on the physical medium, the segments of data making up a frame are converted into transmittable units which are specially tailored for the particular transmission medium on which the Ethernet is operating. This segmenting and conversion procedure is called “encoding”.
The “64B/66B transmission code” is an encoding scheme used in fiber-based 10 Gigabit per second (Gbps) Ethernet systems and described in detail in the IEEE 802.3-2005 standard. This form of encoding performs the following steps before transmission: it segments the transmittable frames and interframe gap data into units of 8 bytes (64 bits) and encodes some control information (see FIG. 1), then scrambles the 64-bit units to obtain direct current (DC) balance and prepends a 2-byte control header (see FIG. 2). The scrambling step uses a Linear Feedback Shift Register (LFSR) or similar mechanism to render the spectral characteristics of the data transmission quite similar to those of random noise. This situation creates inefficiencies in certain topologies, as described in detail below.
The Demand Assignment Multiple Access (DAMA) is a method for resource management in a data communication medium. DAMA enables attachment of a “headend” node (typically belonging to a service provider) and of a number of “secondary” nodes (typically belonging to service subscribers) to the physical medium (e.g. fiberoptic cable). With DAMA (in conjunction with Time-Division Multiplexing or TDM), the headend node controls which network element is allowed to transmit at a given time.
“DOCSIS”, “IEEE 802.16”, and “IEEE 802.3 Ethernet in the First Mile” are examples of networking technologies using TDM-based DAMA mechanisms. A control protocol is typically defined for a specific DAMA network to enable the secondary nodes to indicate their transmission bandwidth requirements (i.e. the “demand”) to the headend and for the headend to signal the “assignments” of transmission slots. All three of the networking technologies mentioned above use DAMA in a point-to-multipoint (P2MP) topology.
In a P2MP network with TDM-based DAMA, the headend node directly controls the flow of data on the downstream (DS) channel (which is received by all of the secondary nodes). The headend also assigns time slots on the upstream (US) channel to particular secondary nodes for their transmissions, as well as “contention slots” in which a secondary node can transmit spontaneously. The upstream transmission of a secondary node within a TDM slot is referred to as a “burst”. The headend device synchronizes its receiver to the incoming signal at the beginning of each burst. Following the end of the burst, the headend device detects that the transmission has completed and resets its receiver in order to detect the next burst.
Additionally, in a P2MP network with TDM-based DAMA, there may exist transmission intervals which are provided by the headend node for new secondary nodes to enter the network. In these intervals (called “discovery” intervals), multiple secondary devices may transmit, although overlapping transmissions may prevent successful reception by the OLT.
An Ethernet Passive Optical Network (EPON) is an Ethernet network implemented on a P2MP topology over fiberoptic media. The headend node is called an Optical Line Terminal (OLT) and a secondary node is called an Optical. Network Unit (ONU). As in other P2MP networks, an EPON OLT transmits over a downstream channel received by all ONUs and assigns transmission slots on the upstream channel to specific ONUs based on their indicated needs for transmission bandwidth. Traditional Ethernet networking technology is oriented to either shared-media (half-duplex Ethernet) or point-to-point media (full-duplex Ethernet). With the introduction of P2MP architecture and DAMA to Ethernet, Ethernet protocols began to be updated for operation in so-called “burst mode”.
The 10-Gigabit-per-second Ethernet Passive Optical Network (10G-EPON) is a revision of the EPON P2MP access link technology. 10 G-EPON incorporates a higher bit rate as well as the 66 bit block encoding mechanism (with scrambling of the 64 bit data payload) described above.
A problem arises in a system which uses an encoding scheme in which the data payload is scrambled. With such an encoding scheme, it is difficult for the data receiver to determine if the transmission is still in progress—because there are no characteristics to differentiate the scrambled data from the noise that follows the data.
As a consequence, in order to determine when to reset its receiver the OLT must either: a) monitor the number of received codewords determined to be invalid after descrambling, or b) monitor the incoming data for invalid information in the non-scrambled portion of each codeword (if such an unscrambled portion exists). The first approach can entail additional latency (especially if Forward Error Correction (FEC) needs to be applied before descrambling). The second approach may (in a channel which tolerates a particular bit error rate) necessitate that a large number of codewords be examined before statistical analysis will support a decision regarding end-of-transmission (as only a few bits of each codeword can be informatively examined).
Consider the case of a scrambled 66 bit block. The first two bits may be 01 or 10, and the rest of the bits are indistinguishable from random noise. Thus exactly half of all randomly occurring 66 bit patterns are legitimate codewords. Moreover, a single bit error in the first two bits can convert a valid codeword to an invalid codeword. Accordingly, in the 10 GBASE-R Ethernet specification, the receiver does not declare loss-of-signal until 16 invalid 2-bit headers have been detected within a range of 64-66 bit blocks (for a worst-case unlock time of 422 ns).
These methods are sometimes sufficient. However, there are some scenarios where reliable, significantly faster detection is required, such as in a P2MP network in which the upstream is shared using burst mode. In particular, for 10 G-EPON, it is desirable that the minimum time between bursts be dominated by the optical components reset time, which may be on the order of 250 ns.
Consequently a new method is needed for the receiver to efficiently detect the end of a transmission in such networks.